Mercaptobenzothiazole (MBT) can be prepared by reacting aniline, carbon disulfide, and sulfur under elevated pressure and at an elevated temperature. At the end of this reaction, the crude reaction mass is discharged into water and MBT is extracted with a dilute alkali metal hydroxide solution whereby MBT is solubilized as an alkali metal salt. U.S. Pat. No. 1,631,871 discloses such as basic process for making MBT.
MBT is useful as a starting material for the production of thiazole derivatives that can be used as vulcanization accelerators. Also, MBT with ethylene glycol can be used as an anticorrosion agent.
Crude MBT is contaminated with various intermediates, unreacted materials, and by-products such as benzothiazole, anilinobenzothioazole, thiocarbanilide, and others, as well as unreacted aniline and sulfur. It is also contaminated with a resinous tarry substance having unknown composition and structure.
A gamut of purification procedures have been proposed in the past for purifying MBT. The Paul U.S. Pat. No. 2,631,153 discloses aeration of the aqueous solution of the alkali metal salt of MBT whereby MBT of improved purity can be obtained. It is disclosed in the Paul patent that during aeration, color of the alkali metal MBT solution gradually changes from dark red to orange or yellow, indicating completion of the aeration step. In the example of this patent, the sodium MBT solution was aerated for 2 hours at 60.degree. C. and then was precipitated.
The Scherhag U.S. Pat. No. 3,770,759 also relates to purification of MBT. This patent discloses a purification process that includes the steps of dissolving the crude melt of MBT in an organic solvent, extracting MBT with a dilute alkali metal hydroxide solution, and precipitating MBT with an acid. The disadvantage of this process resides in the treatment of crude MBT melt with a solvent as opposed to water and the subsequent problems of recovering or controlling the escape of the solvent vapors.
The Sagawa U.S. Pat. No. 3,904,638 also relates to a process for purifying MBT that is obtained by reacting aniline, carbon disulfide, and sulfur. Here, crude MBT is dispersed in an aromatic solvent, the resulting dispersion is filtered to retain particles of MBT which particles are dissolved in an aqueous caustic alkali solution and the solution is then filtered to remove insoluble contaminants. The resulting solution can be used as such in the production of thiazole derivatives or the solution can be acidified with an inorganic acid to obtain solid MBT. The disadvantage of this process resides in the fact that MBT is soluble to some extent in the aromatic hydrocarbon, see Table I, as a result of which, 5% to 10% of MBT can thus be lost.
The Okamoto U.S. Pat. No. 3,804,846 achieves purification of MBT by dissolving crude MBT in an aqueous alkaline solution, reacting the resulting solution with an oxidizing agent under aeration and while being heated, admixing carbon powder and coagulating the oxidation products on the carbon powders suspended in the solution, and separating the carbon powder from the solution whereby products of oxidation are removed therefrom. Examples of suitable oxidizing agents include hydrogen peroxide, sodium or potassium chlorate, sodium or potassium hypochlorite, ammonium perchlorate, and the like.